The present invention relates to a method and a device for crosstalk reduction in a communication link for simultaneous and bidirectional optical transmission.
Cost reduction for subscriber access equipment can potentially be attained if the required photonic devices, laser transmitters and photodetectors, are monolithically integrated on one common semiconductor substrate and if the communication link needs only to use one single optical fibre i.e. not a fibre pair. Such devices have been realised, but they may be limited by crosstalk between the two contra-directional information streams. Sources of crosstalk in these devices include e.g. undesired absorption or electroabsorption of transmitted light in the photodetectors and electrical leakage between the lasers and the photodetectors.
Over the years, several devices and systems for full-duplex transmission over one single optical fibre have been proposed and demonstrated. By way of using distributed Bragg reflector lasers as self-heterodyne laser transceivers, full-duplex 40 Mb/s frequency shift keying (FSK) transmission has been shown to be possible if the known, modulating signal is subtracted from the heterodyned signal, see e.g. R. A. Linke, K. C. Reichmann, T. L. Koch, U. Koren (ATandT Bell Laboratories): xe2x80x9cFull-duplex optical transmission using self-heterodyne laser transceiversxe2x80x9d, IEEE Photon. Technol. Lett., vol.1, pp. 278-280, 1989. In another example, each of the two terminals consists of one single semiconductor optical amplifier, one of which is directly modulated by the data, whereas the other is driven by a 320 MHz electrical subcarrier modulated with 50 Mb/s data, see e.g. P. A. Andrekson, N. A. Olsson (ATandT Bell Labooratories): xe2x80x9cOptical full-duplex transmission with diode laser amplifiersxe2x80x9d, J. Lightwave Technol., vol. 9, pp. 737-740, 1991. In such a configuration crosstalk will be to some extent avoided between the two channels. While the maximum usable bit rate in both these schemes is inherently limited by the time constant associated with the carrier dynamics of forward biased laser structures, the latter is also limited by dispersion occuring in the transmission fibre due to the use of broadband light sources. Residual crosstalk may also be a limiting factor in these two techniques. Another example of an optical communication system, although not bi-directional in its nature, that relies on the use of an electrical subcarrier, may demonstrate simultaneous transmission and detection of 10 Mb/s and optical amplification of 622 Mb/s data in a semiconductor optical amplifier, see e.g. K. T. Koai, R. Olshansky (GTE Laboratories Inc.): xe2x80x9cSimultaneous optical amplification, detection and transmission using in-line semiconductor laser amplifiersxe2x80x9d, IEEE Photon. Technol. Lett., vol. 4, pp. 441-443, 1992. Also in this case the bit rates of transmitted and detected signals are limited by the carrier dynamics of the laser amplifier. At the expense of more complicated photonic devices, that is, using a single-mode laser transmitter in conjunction with a separate, high speed photodetector, the limitations relating to dispersion, speed and to an extent crosstalk, can be partially circumvented. Devices with longitudinally integrated laser and photodetector sections have been described in the following patents: U.S. Pat. Nos. 5,031,188, 5,144,637 and GB-A 2 243 720 to T. L. Koch, H. Kogelnik, U. Koren (ATandT Bell Laboratories).
In another implementation scheme, see, e.g., W. Metzger, J. G. Bauer, P. Clemens, G. Heise, M. Klein, H. F. Mahlein, R. Matz, H. Michel, J. Rieger (Siemens AG): xe2x80x9cPhotonic integrated transceiver for the access networkxe2x80x9d, Proc. 20th European Conference on Optical Communication, post-deadline paper, pp. 87-90, 1994, the laser source and monitor photodetector are integrated in one waveguide branch whereas the receiver photodetector is integrated in another waveguide branch; these two waveguide branches are separated by means of a wavelength-selective structure.
In FIG. 1 an optical communication link for simultaneous and bidirectional transmission is illustrated employing an example of the devices of the aforesaid type, wherein the wavelength corresponding to the energy bandgap of the semiconductor material used in the various sections of the two terminal devices is indicated in units of micrometers. Although such devices present advantages compared to alternative solutions, they may introduce too high crosstalk because of non-negligible optical and/or electrical leakage between the two contra-directional channels. Terminals can be implemented using hybrid or monolithic integration methods. Various material systems are of interest for their realisation, e.g., InGaAsP/lnP and SiO2/Si.
In order to reduce the aforesaid crosstalk, a method and device will be suggested, wherein transmission in the two directions utilises two different optical wavelengths as well as two different electrical carrier frequencies. Transmission in one of the directions may utilise baseband transmission, i.e. a zero frequency electrical carrier. Applications of interest include optically based subscriber access systems and systems for optical interconnections in various information processing systems, such as computers. It is of interest that the equipment for such systems can be produced using relatively simple processes to allow for low cost. In certain applications it is also desirable that these full-duplex optical links can be operated at high bit rates.